I have a seam in 'Shade Smooth' mode that I can't fix
Its caused by the subdivision modifier being used before the mirror modifier. Click and hold on the 6 dots by the top right of the modifier and drag the mirror modifier above the subdivision mod.
I am not 100% sure why having them in the wrong order makes that line appear, but changing the order fixed it for me. Also, it appears the line gets more pronounced the more subdivisions you have.